For example, a vehicle engine generally uses an electromotive force output type gas sensor which takes the exhaust gas discharged from the engine as an object of detection and detects the oxygen concentration. The gas sensor has an electrogenic cell which outputs an electromotive force signal which differs depending on whether the exhaust gas air-fuel ratio is rich or lean. Specifically, when the air-fuel ratio is rich, the gas sensor outputs an electromotive force signal of about 0.9 V and when the air-fuel ratio is lean, the gas sensor outputs an electromotive force signal of about 0 V.
As for this kind of gas sensor, attention has been drawn to the fact that when the air-fuel ratio of the exhaust gas changes to rich or lean, the sensor output changes with a delay from the actual change of the air-fuel ratio. Various techniques have been described to improve this output characteristic.
For example, in the gas sensor control device in Patent Literature 1, a constant current circuit is connected to at least one of a pair of sensor electrodes. When it is determined that a change request to change the output characteristic of the gas sensor has been generated, the direction of constant current is determined according to the change request and the constant current circuit is controlled so that the constant current flows in the determined direction. Thus, the output characteristic of the gas sensor is appropriately controlled by supplying the constant current.
In a gas sensor, the resistance value of the sensor element changes depending on the temperature of the sensor element. Specifically, when the engine is started in the cold or when the exhaust gas temperature decreases with fuel cut to the engine, the element resistance increases with the decrease in the temperature of the sensor element. In this case, as the element resistance increases, the voltage applied to the sensor element increases even under the condition that a constant current flows. Because of this, the accuracy of determination about a rich or lean air-fuel ratio may decrease. The sensor element corresponds to an electrogenic cell. The resistance value of the sensor element is also called the element resistance.
More specifically, in the microcomputer which receives electromotive force output from the gas sensor, a first threshold which is on a richer side than the stoichiometric value (0.45 V) and a second threshold which is on a leaner side than the stoichiometric value (0.45 V) are predetermined. For example, the first threshold is set to 0.6 V and the second threshold is set to 0.3 V. When the electromotive force output is larger than the first threshold, the air-fuel ratio is determined as rich and when the electromotive force output is smaller than the second threshold, the air-fuel ratio is determined as lean. In this case, when the voltage applied to the sensor increases due to an unintentional change in the element resistance, air-fuel ratios which are determined as rich (or lean) would vary widely, resulting in decrease in the accuracy of air-fuel ratio determination. This problem arises because the air fuel ratio is determined as rich or lean not in the region of the gas sensor output characteristic where the electromotive force output rapidly changes, but its stable region which is on a richer or leaner side than the rapid change region. Therefore, in ensuring the accuracy of air-fuel ratio determination, there is room for improvement.